I think that your success in always finding sites is based on the fact that you only stay for a few days. Lots of CG's have sites in two - three day slots. I saw it just last week when the popular CG I was in had sites- nice ones at that, open for 2 - 3 days while we were there. The people who pulled in next to me noticed this but were clueless - the lady exclaimed: "I can't understand why it is so hard to get in here - I see lots of open sites!"
When I want to go camping, I want to stay longer - even longer if the drive is over 5 hours. That is harder to find, especially when booking includes a weekend. If I was on the road constantly, and needed a place to lay up for a day or two, I would expect to have your experience of not necessarily needing a reservation. But for most - who camp on vacation time, a reservation is needed.